Summary

• Fixed ParameterNodeAtInstant.__getitem__() to convert pandas StringArray to numpy array before using for fancy indexing

Problem

In pandas 3.0, string columns use StringDtype by default, producing StringArray objects. When a StringArray is passed to ParameterNodeAtInstant.__getitem__(), it fails with:

TypeError: unhashable type: 'StringArray'

This is because StringArray is not hashable and can't be used as a dict key in self._children[key].

Solution

Added the same check that already exists in VectorialParameterNodeAtInstant.__getitem__():

if hasattr(key, "__array__") and not isinstance(key, numpy.ndarray):
    key = numpy.asarray(key)

This converts pandas arrays (like StringArray) to numpy arrays before checking for fancy indexing.

Test plan

• Added regression test test_parameter_node_at_instant_getitem_with_string_array
• Verified test fails before fix
• Verified test passes after fix
• All pandas3 compatibility tests pass

Fixes #429

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